For protection of signal transmission by coded frequency-shift keying, two frequencies, for example f1, f2 are allocated to each command to be transmitted or to each command combination to be transmitted. The two frequencies are then transmitted for a specific time period T:
. . . , f1, f2, f1, f2, f1, f2, f1, f2, f1 
A command change to be carried out from a command A, which is associated with the frequencies f1, f2, to a command combination A+B, which is associated with the frequencies f3, f4, can be done as follows:
. . . , f1, f2, f1, f2, f3, f4, f3, f4, . . .
If, for example, the frequencies f2 and f3 were now to be allocated to a command C, the command sequence would accordingly be A, C, A+B and not, as desired, A, A+B. This leads to errors, for example in the form of spurious initiations.
There are various variants or options for a command change for the command changes as described above.
A first variant or option for a command change is referred to as a single frequency change. In the case of a single frequency change, a first frequency f1,BK1 or a second frequency f2,BK1 in the preceding command combination BK1 in the preceding command combination BK1 matches a first frequency f1,BK2 or a second frequency f2,BK2 in the subsequent command combination BK2. By way of example, the following frequency chain which represents a sequence of commands exhibits a single frequency change such as this from a preceding command combination BK1 with a frequency pair (f1, f2) to a subsequent command combination BK2 with a frequency pair (f2,f3):
. . . f1, f2, f1, f2, f3, f2, f3, f2, . . .
A single frequency change is applied analogously for a command B.
A second option is referred to as a double frequency change. In the case of a double frequency change, both the first frequency f1,BK1 and the second frequency f2,BK1 change to the subsequent command combination BK2 when a command change takes place from the preceding command combination BK1. This double frequency change can be carried out via an intermediate step. By way of example, the following frequency chain, which represents a sequence of commands, exhibits a double frequency change such as this from a preceding command combination BK1 with a frequency pair (f1,f2) to a subsequent command combination BK2 with a frequency pair (f3,f4) via an intermediate step (f2,f3):
. . . f1, f2, f1, f2, f3, f4, f3, f4, f3, f4, . . .
A double frequency change is applied analogously for a command B.
The change process described above could also be carried out via the frequency pairs (f1,f3); (f1,f4); (f2,f4). This means that four intermediate steps are possible on the basis of the combination for a double frequency change.
In order to maintain the correct command sequence, it would be possible not to allocate any command combinations to specific frequency pairs. In this specific example, this means that no command or command combination is allocated to the frequency pairs (f1,f3); (f1,f4); (f2,f4). However, this can be disadvantageous if, for example, there are a large number of commands, since the frequency range can then be very wide in order to cover the large number of different combinations.
Another solution art is to insert a pause between the individual command combinations. However, this can be disadvantageous because the total time for transmission of command combinations in consequence can rise considerably, and rapid transmission of command combinations is no longer guaranteed.